Braking structure for heavy duty vehicles, such as trucks and trailers, often include a brake actuator which is of the spring operated fail-safe type. The actuator is in the form of an expansible chamber motor and a piston and piston rod assembly is reciprocal within a cylinder, while a helical spring within the cylinder biases the piston and piston rod in a braking direction. Displacement of the piston is controlled by a pressurized medium, such as air or hydraulic fluid, introduced within the cylinder exerting a force upon the piston counter to that imposed by the spring. Braking is achieved by reducing the medium pressure to permit the spring to produce the braking force, and pressurizing the cylinder will retract the piston and rod into a nonbraking condition.
In prior art brake actuators of the aforedescribed type the piston is usually of a cast and machined construction, and is expensive to produce. Such conventional construction, in addition to its cost, adds significant weight to the actuator, and as a plurality of actuators are mounted upon a vehicle the actuators' weight is accumulative.
The actuators of the aforedescribed type also employ a cylinder end cap or head remote from the end of the expansible chamber motor through which the piston rod extends. The compression spring bears against this end cap, and in view of the high axial forces exerted upon the end cap a dependable and high strength interconnection between the end cap and cylinder must be maintained. In the past, a plurality of screws, often eight in number, have been employed to interconnect the end cap to the cylinder. While such an interconnection provides the necessary strength and resistance to axial force, the installation of the screws is time consuming, expensive, requires orientation between the end cap and cylinder, and may necessitate special equipment during installation.
It is an object of the invention to provide a brake actuator of the expansible chamber motor type wherein a majority of the components are formed of sheet metal, including the piston, and wherein acceptable strength and wear characteristics are achieved.
A further object of the invention is to provide a brake actuator of the expansible chamber motor type utilizing a sheet metal piston of a drawn construction wherein a piston rod is attached to the piston by an interference press fit, and wherein a wire ring forced upon the piston periphery retains the piston seal adjacent the cylinder.
Another object of the invention is to provide a vehicle brake actuator of the expansible chamber motor type wherein a heavy duty compression spring provides the braking force, and wherein the spring engages a cylinder end cap which is assembled to the cylinder by a single retainer ring.
A further object of the invention is to provide an expansible chamber motor brake actuator of the spring powered type employing an end cap retainer ring wherein the spring force is employed in maintaining the retainer in operative position.
In the practice of the invention the brake actuator comprises an expansible chamber motor having a sheet metal cylinder. At one end of the cylinder a seal is defined slidably receiving a piston rod, and at the other cylinder end a head or end cap is located which is removably mounted on the cylinder by a contractable retaining ring. A piston is mounted upon the piston rod within the cylinder chamber, and a heavy duty compression spring interposed between the end cap and piston biases the piston and piston rod in a braking direction. An inlet for the pressurized medium is defined in the cylinder wall for pressurizing the cylinder chamber on the opposite side of the piston with respect to the spring to permit hydraulic fluid, or compressed air, to shift the piston in a spring compressing, non-braking, direction.
In accord with the invention the piston is of a drawn sheet metal construction. The piston includes a central recess having a diameter less than that of the piston rod whereby the piston rod is pressed into the piston recess to complete the assembly of the piston and piston rod, and an abutment shoulder is defined in the piston recess to limit the extent of insertion of the rod into the recess, and aids in the transference of axial forces between the piston and piston rod.
Further, the piston is provided with sealing ring structure at its periphery, and the sealing ring structure is maintained in position by a circular metal ring which is pressed upon the piston periphery in an interference fit. In this manner, the sealing ring structure is retained upon the piston in a most economical manner without requiring machining or additional operations to the piston.
The cylinder end cap is also of a sheet metal construction and is assembled to the cylinder by a split contractable ring. A plurality of openings are defined within the cylinder wall adjacent its "open" end, and when assembling the actuator the end cap is inserted into the cylinder open end inwardly of the cylinder openings. Thereupon, a split retaining ring of a generally circular configuration which includes a plurality of radially extending projections is contracted and inserted within the cylinder such that the projections are received within and extend outwardly through the cylinder openings when the ring expands, and the end cap is permitted to move back against the retaining ring once the ring has expanded due to the alignment of the ring projections with the cylinder openings. The configuration of the end cap adjacent the retaining ring is such that the end cap maintains the retaining ring in an expanded condition preventing radial contraction of the ring, and assuring a firm and foolproof mounting of the end cap on the cylinder which will not loosen from vibration and is relatively tamperproof.
In an embodiment of the invention a permanent "back-off" bolt is mounted upon the piston rod for maintaining the spring between the end cap and piston in a compressed state wherein the piston rod is retracted for shipping and installation purposes. In contradistinction to the usual back-off bolt operation wherein the bolt is only installed in the piston rod during installation, or for the purpose of manually compressing the piston spring, the bolt is permanently mounted within and carried by the piston rod. The bolt includes an enlarged head, and the end cap has an opening having a greater transverse dimension than that of the bolt head wherein the bolt and its head freely move through the end cap opening during the normal actuator operation. For shipping and installation purposes, an insert is located between the bolt head and the end cap opening when the piston rod is retracted, and as the insert is of greater dimension than the end cap opening the insert will prevent the bolt head from passing through the end cap opening and the bolt will retain the piston rod in a retracted condition. After installation, pressurization of the cylinder further compresses the piston spring permitting the insert to be removed from between the bolt head and the end cap, and normal operation of the brake actuator is possible while the back-off bolt remains in the piston rod.